Plasma has long been employed in plasma processing equipment to process substrates (e.g., semiconductor substrates, flat panel substrates, nano-machinery substrates, and the like) into useful devices (e.g., integrated circuits, flat panels, nano-machines, and the like). Up to now, plasma ignition has taken place at a relatively high chamber pressure, for example at 120 mTorr for a 2300 Exelan®-series plasma processing system, which is available from Lam Research Corporation of Fremont, Calif.
One of the reasons for requiring a high chamber pressure for plasma ignition is that plasma ignition has been unreliable at lower pressures. This is due to a low reactor gap, the use of relatively low RF frequencies for plasma generation and the fact that the RF energy is being coupled into the plasma capacitively. However, the high chamber pressure at which plasma is ignited in the prior art creates certain processing challenges. For example, a high chamber pressure may reduce etch directionality and contribute to higher level of polymer formation on the substrate surface, leading to etch inconsistencies such as etch stop and anomalous RF plasma formations.
As technology progresses and etch requirements become more exact, the reduced etch directionality and the higher level of polymer formation become significant disadvantages. This statement is particularly true for processes that require a low process pressure during the plasma step, e.g., below 50 mTorr in some cases. The pressure can be lowered during a processing step (e.g., to about 60 mTorr) but the requirement of a high pressure ignition step still adds an additional step to the overall etch process, which disadvantageously increases the overall etch time. Furthermore, the higher pressure ignition step may require a challenging transition phase in which the process pressure will have to be reduced to its target magnitude. In addition, excessive polymer may be formed on surfaces during the high pressure phase leading to problems such as etch stop on the wafer or anomalous plasma formation.